Creating a bug from the webkit-dev discussion. It seems there's consensus on doing this. The main reasons are that without a robust way to handle tracking, ad networks and other sites are resorting to redirects and busy loops in unload handlers after image loads. This hurts the user since it slows down their navigation and consumes battery on mobile/laptop devices. Adding ping doesn't reduce the privacy, since it's something that's already done using hacks. This should improve privacy since user agents can then expose options to the user to disable this feature, just like for referral headers.
This is part of HTML5: http://www.whatwg.org/specs/web-apps/current-work/#hyperlink-auditing
Note: once we do this, there are a few people who think we should implement window.sendPing() as well. The reasons are that <a ping> won't work for iframes, which is how it's used by ad networks.